This invention relates to inverter circuits.
Inverter circuits for converting DC input voltage to higher-voltage output to charge a capacitive load, are known and find application, for example, in the powering of xenon flash-discharge tubes. Xenon flash-discharge tubes are used to produce repetitive short-duration flashes of light for beacon purposes in giving a visual signal or warning, and in this respect may be used in burglar-and fire-alarm systems and on police, ambulance, fire-service and other vehicles.
In many applications and potential applications of xenon tubes in this way, the voltage of the available DC power supply may have any of a number of nominal values, and in any particular case may be liable to vary significantly from that nominal value. For example, the beacon may be for use on a vehicle having a 12-volt, 24-volt or other battery, and the battery-voltage may vary significantly from the nominal value according to the state of charge of the battery and whether the vehicle""s engine is running. It is not in general simple to provide for satisfactory operation of xenon tubes in these circumstances using known inverter circuits, since the output of the inverter circuit is too dependent on the power-supply voltage. Moreover, the majority of xenon tubes operate satisfactorily only within a narrow range of applied voltage.
It is one of the objects of the present invention to provide a form of inverter circuit that may be used in the above circumstances to power a xenon tube satisfactorily throughout a wide range of DC supply voltages.
According to the present invention there is provided an inverter circuit for charging a capacitive load, in which switching means is connected in series with inductance across DC-input terminals of the circuit to switch cyclically from an ON state to an OFF state for charging the capacitive load incrementally from fly-back in the inductance, the switching means being held in its OFF state during each cycle of operation by feedback of the fly-back voltage, wherein the circuit includes means for deriving a voltage that is dependent on the voltage across the capacitive load and for applying the derived voltage to counteract the feedback such as thereby to interrupt the cyclic operation of the circuit.
Switching of the switching means between its ON and OFF states may be regulated by a transistor device such that the switching means has its ON state while the transistor device is OFF and its OFF state while the transistor device is ON. The transistor device may be turned ON so as to initiate the fly-back of the cycle, in response to build up of current in the inductance, and it may be maintained ON during the fly-back by the feedback. In these circumstances, the derived voltage dependent upon the voltage across the capacitive load, may be applied to the transistor device to counteract the feedback by holding the transistor device OFF during the fly-back.
The voltage to which the capacitive load is charged may be limited by the counteracting effect of the derived voltage to a level that is substantially independent of the voltage of the DC-supply connected to the input terminals. This can be achieved with the inverter circuit of the invention simply by choice of the relative values of two resistors, one in the feedback path and the other used for derivation of the voltage used in counteraction of the feedback. Moreover, by adjustment of the relative values of these resistors, it is possible even to achieve limitation of the voltage to which the capacitive load is charged, to a level that is lower the higher the voltage of the DC-supply connected to the input terminals.